In the memory circuit disclosed in U.S. Pat. No. 6,744,669 B2 Publication, a boosted voltage that is increased from a power supply voltage VCC and required for rewriting or erasing operation is supplied to an array of memory cells by a boost circuit 200 as shown in FIG. 2 of U.S. Pat. No. 6,744,669 B2 Publication. The boost circuit 200 is a so-called booster type DC-DC converter circuit having an inductive element 210, a switching transistor T1, a diode D1 and a capacitor C2. Cyclic control signals make the switching transistor T1 periodically conductive. When the switching transistor T1 is in its conductive state, a current flows from the power supply VCC to ground potential through the inductive element 210 and the switching transistor T1. Thereby, the inductive element 210 builds up energy. When the switching transistor T1 becomes nonconductive, the drain terminal voltage of the switching transistor T1 rises and the energy is transferred to the capacitor C2 through the diode D1. This causes the terminal voltage of the capacitor C2 to increase from the power supply voltage VCC so that a boosted voltage is fed from the output terminal Output.
Herein, the diode D1 is constructed by wire connection of the gate terminal and drain terminal of an NMOS transistor as shown in the drawing. The drain terminal of the NMOS transistor functions as the anode terminal of the diode whereas the source terminal of the NMOS transistor as the cathode terminal of the diode. The drain terminal is connected to the inductive element 210 whereas the source terminal is connected to the output terminal Output, with the direction from the inductive element 210 to the output terminal Output serving as a forward direction. When the switching transistor T1 is conductive, the diode D1 is reversely biased to prevent a reverse flow of the boosted voltage from the output terminal Output. When the switching transistor T1 is nonconductive, voltages at the switching transistor T1 and the drain of the NMOS transistor are increased by the energy released from the inductive element 210 so that the diode D1 is forwardly biased and a boosted voltage is outputted to the output terminal Output.